This invention relates to a process for producing a precursor of a silicon-containing polyimide and a process for curing a solution containing the same.
Heretofore, polyimide resins have often been used for heat-resistant films, coating agents, adhesives for making the resins composite with inorganic materials or metallic materials, etc. In this case, if the inorganic materials are silicon-containing compounds such as glass, a number of copolymers containing silicon compounds in the precursors of polyimides have been proposed as a means for improving the adhesion of polyimides.
For example, Japanese patent application laid-open Nos. Sho 57-143328/1982, Sho 58-7473/1983 and Sho 58-13631/1983 disclose a process wherein a polyimidesiloxane copolymer is prepared from a polyimide precursor obtained by replacing a portion of a diamine as a raw material by a polysiloxane terminated at both the ends thereof by a diamine. In this case, improvement in the adhesion to a certain extent has been observed, but instead a problem has been raised that the polymerization degree of the copolymer is reduced with the increase in the siloxane content in the copolymer.
Further, Japanese patent publication Nos. Sho 58-18372/1983, Sho 58-32162/1983 and Sho 58-32163/1983 disclose a process wherein a suitable carboxylic acid derivative such as a tetracarboxylic acid dianhydride is reacted with a diamine to form a polyamide carboxylic acid having a terminal group such as an acid anhydride, followed by reacting one mol of this polyamide carboxylic acid with at least two mols of an aminosilicon compound at a temperature of -20.degree. C. to +50.degree. C. to obtain a silicon-containing polyamide carboxylic acid prepolymer, using this prepolymer, as it is, i.e. without any imidization to obtain an organosilicon-modified polyimide precursor, or imidizing the prepolymer by chemically cyclizing (imidizing) the prepolymer in the presence of a dehydrating agent under a mild condition (at a low temperature, preferably at 50.degree. C. or lower, particularly at -20.degree. C. to +25.degree. C.), to obtain an organosilicon-modified polyimide precursor, and heating these precursors in the presence or absence of a silanediol or siloxanediol in solution state to complete its imidization and also effect cross-linking and thereby obtain the objective polyimidesiloxane. According to the process, however, adhesion to silicon compounds has been improved to a certain extent, but when the polyimide obtained according to the process is subjected to heat history over a broad temperature range, the resulting adhesion is inferior. In order to improve the adhesion, the thermal expansion coefficient of the polyimide should be lowered.
On the other hand, Japanese patent application laid-open Nos. Sho 60-32827/1985, Sho 60-250031/1985, Sho 61-60725/1986 and Sho.61-241325/1986 propose a biphenyltetracarboxylic acid or pyromellitic acid polyimide having a low thermal expansion coefficient. However, the measurement of the coefficient of these polyimides has been carried out at temperatures of 300.degree. C. or lower, but the values rapidly increase at 400.degree. C. to 500.degree. C. Further, adhesion thereof with inorganic materials is inferior.
In general, the thermal expansion coefficients of polyimides at room temperature to 450.degree. C. are at the least about 3.times.10.sup.-5 deg.sup.-1, whereas those of metals or inorganic materials are of an order around 10.sup.-5 to 10.sup.-6 deg.sup.-1. For example, stainless steel of type 410 (annealed): 11, cast iron: 12.1, industrial purified copper: 17, polycrystalline glass: 5.8, silica glass: 0.5 to 0.8, borosilicate glass: 3.0 to 6.0; and porcelein: 4.5, each .times.10.sup.-6 deg.sup.-1, see Handbook of Chemical Engineering (edited by Japan Chemical Engineering Association, issued by Maruzen Company). Accordingly, when polyimides are made composite with these inorganic materials to prepare composite materials or used as adhesives for the materials, it is necessary to improve their adhesion and at the same time reduce their thermal expansion coefficiencies and thereby approach those of the materials. The present inventors have made extensive research for the above purpose, and as a result have achieved the present invention and also the present invention is the one having improved a process previously proposed by some of the present inventors (Japanese patent application laid-open No. Sho 61-287926/1986) in the aspect of the thermal expansion coefficient.